U.S. patent application Ser. No. 09/076,494, filed May 12, 1998, and entitled, "SYSTEM AND METHOD FOR FREE SPACE OPTICAL COMMUNICATIONS," assigned to the present assignee by the same inventors, and hereby incorporated by reference in its entirety, describes a method by which next generation satellite communication systems could achieve extremely high data rates for direct intersatellite, satellite-to-ground, and ground-to-satellite communications over extremely large line-of-sight distances using optical technology. The application achieved a technological advance by circumventing various shortcomings, such as common with optical data communication technology that followed the development of terrestrial fiber optic networks and concentrated solely on the transmission of digital information, typically using a pulse-pulse modulation (PPN), on-off keying (OOK) format, or wavelength-division multiplexing (WDM) to increase the information rate in optical channels. Those techniques did not fully exploit the advantages of optical communication technology for high data rate space based applications.
The invention disclosed in the '494 application circumvented those shortcomings by electrically combining a number of data sources, digital or analog, using a frequency-division multiple access theme, and using this signal as a wide band modulating signal to alter the phase of a single optical carrier. The constant envelope of phase modulation was advantageous as compared to amplitude modulations (OOK), (PPM) for simplifying detection schemes as is well known in communications. At the receiving terminal, the carrier is coherently demodulated and the individual electrical signals recovered using filtering of amplification. The invention described in that application, unlike known prior art, allows digital and analog signals to simultaneously share a single optical carrier.
The invention of the '494 application allows both analog and digital data to be transmitted simultaneously on a phase modulated optical communication signal to a receiver, such as for intersatellite and satellite-to-ground communications. Because the optical carrier signal is phase modulated, the problems associated with moving targets and changes in distances between the targets, e.g., satellites and/or ground stations and satellites, are reduced. A constant envelope type of modulation, i.e., phase modulation, is used instead of the more conventional intensity modulation, which changes the amplitude of the signal. As a result, no auxiliary or pilot signal is necessary. Additionally, the phase modulated signal is readily adapted for non-mechanical steering, which decreases any payload weight for communications equipment, requires less fuel and decreases acquisition times.
Typically, a mixture of analog, digital or RF signals are each passed through a mixer where respective signals are up converted into a unique signal slot or channel. The frequencies then are combined to form the broad band frequency division multiplexed signal. A laser generates an optical carrier signal. An electro-optic modulator phase modulates the optical carrier signal with the multiplexed signal to produce a phase modulated optical communication signal.
A receiver is positioned, such as in a satellite, to receive the phase modulated optical communications signal. The receiver comprises a demodulator for demodulating the phase modulated optical communications signal back into the original broad band frequency division multiplexed signal. A demultiplexer (e.g., filter) allows demultiplexing of the broad band frequency division multiplexed signal into the plurality of communication signals comprising the frequency division multiplexed signal.
A plurality of digital communication signals are generated and analog modulated onto an optical carrier using an electro-optic technique. The electro-optic modulator can preferably comprise a Mach-Zender electro-optic modulator. An antenna can receive communication signals to be multiplexed with a receiver, such as in a satellite, and can be connected through the frequency division multiplexer for receiving analog communication signals generated by a remote source. The electro-optic modulator preferably generates an optical carrier signal wavelength of about 1,550 nm. This wavelength is preferable because erbium-doped fiber amplifiers can be used at this wavelength for amplifying the phase modulated optical communication signals.
However, a receiving satellite may be positioned behind the horizon or otherwise, improperly positioned, and thus, a transmitting satellite would not be able to transmit the signal adequately. It is therefore essential that some type of repeating station be considered for the two satellites. Additionally, because both analog and digital data, as well as various different data, are transmitted on the signal, it may be desirable to pull off part of the signal, and then transmit the balance of the signal to a receiving satellite. This intermediate satellite that would pull off part of the signal and then transmit the balance or remainder of the signal acts as an optical non-processing transponder, which would not demodulate the signal for pertinent data and information, but only retransmits the signal after amplifying, such as similar to a normal RF non-processing satellite transponder repeater.